Generally, a heat-sensitive transfer recording medium is referred to as a thermal ribbon which is an ink ribbon used in heat-sensitive transfer type printers, and has a heat-sensitive transfer layer on one surface of a base material, and a heat-resistant slippage layer (back coat layer) on the other surface of the base material. Here, the heat-sensitive transfer layer is a layer of ink, and the ink is sublimed (sublimation transfer type) or melted (melt transfer type) by heat generated by a thermal head of a printer to be transferred to a transfer-target object side.
At present, since the sublimation transfer type, among the heat-sensitive transfer types, enables high performance printers to easily form various images in full color, the heat-sensitive transfer type is widely used for do-it-yourself printing for digital cameras, cards such as identification cards, output objects for amusement, etc. In addition to diversification of such use applications, the demands for reduction in size, faster speed, cost reduction, and durability of obtained printed objects have become large. As a result, there has been quite widespread use in recent years of heat-sensitive transfer recording media, having a plurality of heat-sensitive transfer layers with protective layers and the like disposed so as not to overlap, for providing durability to printed objects on the same side of a base material sheet.
Under such circumstances, associated with diversification of use applications and widespread use and as higher printing speed is achieved by printers, a problem arises where sufficient print density cannot be obtained with a conventional heat-sensitive transfer recording medium. Thus, in order to increase transfer sensitivity, attempts have been made for improving transfer sensitivity during printing by reducing the thickness of the heat-sensitive transfer recording medium. However, there are problems such as generation of wrinkles and in some cases generation of fractures due to heat, pressure, etc., when manufacturing the heat-sensitive transfer recording medium or when printing therewith.
In addition, there have been attempts to improve print density and transfer sensitivity during printing by increasing the ratio (dye/binder) of dye with respect to resin in a dye layer of a heat-sensitive transfer recording medium. However, increasing the amount of dye not only increases cost, but also results in so called scumming such as resulting in a hue different from a specified color when, during manufacturing, one part of the dye shifts (set-off) to a heat-resistant slippage layer of a heat-sensitive transfer recording medium in a wind-up state, the shifted dye is, during roll-back thereafter, retransferred (secondary set-off) to a dye layer of another color or to a protective layer, and the tainted layer is thermally transferred to a transfer-target object.
Furthermore, attempts have been made to increase energy on not only the heat-sensitive transfer recording medium side but also on a printer side when forming images. However, not only power consumption increases but the lifespan of a thermal head of the printer becomes shorter, and so-called abnormal transfer of fusing of a dye layer and transfer-target object occur easily. As a response, when a large amount of mold releasing agent is added to a transfer-target object or a dye layer for preventing abnormal transfer, blurring of an image and scumming occurs.
Several methods have been proposed to solve such problems. For example, Patent Literature 1 proposes a thermal transfer sheet including, between a base material and a dye layer, an adhesion layer containing a polyvinyl pyrrolidone resin and a modified polyvinyl pyrrolidone resin.
In addition, Patent Literature 2 proposes a thermal transfer sheet including, between a base material and a dye layer, an adhesion layer including colloidal inorganic pigment ultrafine particles, and a polyvinyl alcohol resin or a polyvinyl pyrrolidone resin which is a thermoplastic resin.
Furthermore, Patent Literature 3 proposes a thermal transfer sheet including, between a base material and a dye layer, a foundation layer including colloidal inorganic pigment ultrafine particles and a vinylpyrrolidone-vinyl acetate copolymer.
Disposing a layer including a specific material between a base material and a dye layer as described above improves transfer sensitivity. As the transfer sensitivity is improved, it becomes possible to reduce the thickness of the dye layer, resulting in reduction in the total amount of the dye and reduction in cost cut. However, there are problems such as generation of wrinkles and in some cases generation of fractures due to heat, pressure, etc., when printing with the heat-sensitive transfer recording medium.
Wrinkles when printing with the heat-sensitive transfer recording medium may occur due to adhesion of the base material and the thermal head when slippage of the heat-resistant slippage layer is insufficient. In addition, if slippage of the heat-resistant slippage layer is largely different between low-energy printing and high-energy printing, in cases such as, for example, when a printed part and an un-printed part coexist on the same image, wrinkles may occur due to a difference in friction between the thermal head and the heat-resistant slippage layer.
In order to solve such problems, for example, Patent Literature 4 proposes a thermal transfer sheet configured to prevent wrinkles to occur during printing by improving slippage during high-energy printing by adding, to the heat-resistant slippage layer, a silicone modified resin, a metallic soap, and a filler component.
Furthermore, an aqueous thermal transfer image-receiving sheet having formed thereon an aqueous-receiving layer tends to have strong adhesivity on a side of the image-receiving sheet, and it is confirmed that the required releasability is different depending on whether high energy is applied (high density) or intermediate energy is applied (intermediate density) during printing from a thermal head.
For a conventional oily image-receiving sheet, since it is sufficient when releasability for high density printing is ensured, the thermal transfer sheets disclosed in, for example, Patent Literature 1 to 3 can handle printing to a certain degree. However, when an aqueous image-receiving sheet is used, since adhesion tends to occur even for intermediate density printing, a thermal transfer sheet that can sufficiently handle intermediate density to high density printing is necessary.
Thus, Patent Literature 5 proposes a thermal transfer sheet having a dye layer that contains a dye, a resin binder, and a mold releasing agent at a specific amount with respect to the resin binder, and whose water content is adjusted to be 2.5% or lower.